This is the problem with abstinence-only "education". Before a man comes, there's already fluid leaking (pre-ejaculate), and it often will have enough sperm to knock a woman up if she's at the right time in her cycle. Not to mention if a guy is drunk, he may not do so in time.

I'm willing to be OP's girlfriend just didn't want to have sex with her drunken boyfriend and made up a stupid excuse.